This application claims the priority of Korean Patent Application No. 2003-781 filed on Jan. 7, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to an edge light type backlight unit using a light guide panel (LGP).
2. Description of the Related Art
Since a light receiving type flat panel display device such as a liquid crystal display device forms an image not by emitting light by itself but by receiving light from the outside, the image cannot be viewed in a dark place. Thus, a backlight unit for emitting light is installed at a rear surface of the light receiving type flat panel display device.
The backlight units can be classified into direct light types and edge light types according to the type of arrangement of a light source. The edge light type light source can use a linear light source and a point light source as a light source. As a typical linear light source, there is a cold cathode fluorescent lamp (CCFL) in which electrodes are installed inside a tube at both end portions. A light emitting diode (LED) is a typical linear light source. The CCFL can emit a strong white light and obtain a high brightness and a high uniformity. Also, the CCFL can enable a device of a large size design. However, the CCFL is disadvantageous since it is operated by a high frequency AC signal and has a narrow operational temperature range. The LED exhibits a lower performance in brightness and uniformity compared to the CCFL, but is operated by a DC signal and has a long life span and a wide operational temperature range. Further, the LED can be manufactured in a thin shape.
FIG. 1 is a perspective view illustrating a conventional edge light type backlight unit using a point light source. FIG. 2 is a sectional view illustrating the edge light type backlight unit shown in FIG. 1.
Referring to FIG. 1, three LEDs 20 are installed at an edge 11 of a light guide panel 10 as point light sources. A holographic pattern 30 for making the light emitted from the LEDs 20 proceed through a light exhaust surface 12 is formed on a lower surface of the light guide panel 10. Although not shown in the drawing, there is a diffusion panel for diffusing the light exhausted from the light exhaust surface 12 and a prism panel for improving brightness of the diffused light in a direction perpendicular to the light exhaust surface 12.
The LEDs 20, as shown in FIG. 3, emits light in a range between 0–±90°. The light emitted from each of the LEDs 20 is incident on the light guide panel 10 through the edge 11 and then input to the holographic pattern 30. The holographic pattern 30 having a diffraction grid structure formed perpendicular to an optical axis 21 changes the incident light to a surface light and makes the surface light proceed through the light exhaust surface 12 which is an upper surface of the light guide panel 10. The holographic pattern 30 can emit light at the highest efficiency when the light is incident on the holographic pattern 30 at an angle of 90°. Also, as the distribution of an incident azimuth angle of the light incident on the holographic pattern 30 decreases, a uniform brightness can be obtained at the light exhaust surface 12. If the brightness of the light exhaust surface 12 is not uniform, a screen appears to be smeared. In a narrow range of about 1 centimeter, a change in brightness of about 0.9 is detected as a smear. However, when the brightness changes gradually from the central portion of the screen to an edge portion thereof, a change in brightness of about 0.8 is not detected as a smear. Thus, a uniformity of brightness over 0.8 is needed. In particular, to obtain a quality image, a uniformity of brightness over 0.9 is needed.
FIG. 4 shows the distribution of the output of light by the conventional backlight unit shown in FIG. 1. The light guide panel 10 is divided into three portions, that is, a near portion 40, a middle portion 50, and a far portion 60, sequentially from the edge 11 where the LEDs 20 are installed, where the distribution of the output of light is shown. Referring to FIG. 4, the middle portion 50 and the far portion 60 have a wider light output distribution compared to the near portion 40.
FIG. 5 is a graph showing the brightness at the light exhaust surface 12 by the edge light type backlight unit shown in FIG. 1. In the graph, the vertical axis indicates brightness and the horizontal axis indicates FWHM (full width half maximum) showing a light exhaust angle at the light exhaust surface 12. Three curves C1, C2, and C3 from the left side indicate the brightness of the near portion 40, the middle portion 50, and the far portion 60, respectively. Referring to FIG. 5, it can be seen that the brightness of the near portion 40 is greater than those of the middle portion 50 and the far portion 60. The FWHM of the near portion 40 is 20°/20° while those of the middle portion 50 and the far portion 60 are 20°/35° which are relatively wider than that of the near portion 40. In 20°/35°, the first angle “20°” and the second angle “35°” denote FWHMs in an X direction and a Y direction, respectively.
The irregularity of brightness exists because the distribution of an incident azimuth angle of the light incident on the holographic pattern 30 is different in each of the near portion 40, the middle portion 50, and the far portion 60 so that an efficiency in the light emission by the holographic pattern 30 and the distribution of an exhaust azimuth angle of the exhaust light are different in the three portions.